Electrical components mounted on automotive vehicles such as car navigation systems have increased in recent years. A current drain on car batteries has become too large accordingly, and a peak current has reached to several hundred amperes. A variety of technologies improving fuel economy have therefore applied to cars. An engine control system, which is one of the technologies, stops to operate a battery-charging generator under acceleration and operates the generator under deceleration. The engine control system requires precision detection capability of a battery current in order to control the battery charging properly.
FIG. 15 shows a conventional current sensor 10 for measuring a large current such as a battery current. The sensor 10 has a C-shaped magnetic core 20, a current bus bar 12 and a magnetic sensor 14. The core 20 has a center opening where the bar 12 passes through. A gap Ga1 is formed between both end surfaces of the core 20. The magnetic sensor 14 is disposed in the gap Ga1. The magnetic sensor 14 detects magnetic flux density in the gap Ga1 generated by a current flowing through the bar 12. Then, the magnetic sensor produces signals corresponding to the magnetic flux density. The sensor 10 receives the signals, and thereby can measures a current.
Current sensors such as the sensor 10 are disclosed, for example, in Japanese Patent Application Publication No. H14-286764, Japanese Patent Application Publication No. H14-303642, Japanese Patent Application Publication No. H15-167009, and Japanese Patent Application Publication No. 2002-350470.
A current sensor disclosed in Japanese Patent Application Publication No. H14-286764 uses magnetoimpedance devices (i.e., MI device) as magnetic sensing elements. In the sensor, sensitivity of a weak direct current is improved by applying alternating current to the MI devices.
A current sensor disclosed in Japanese Patent Application Publication No. H15-350470 uses two Hall effect ICs as magnetic sensing elements. One IC is for a large current, and the other IC is for a small current. The sensor automatically switches on and off between the two ICs according to an amount of current.
However, the sensor 10 shown in FIG. 15 measures a certain amount of current, even when no current flows. The measurement error of current is caused by magnetic hysteresis effect in the core 20, which is ferromagnetic. Specifically, when a large current flows through the bar 12, the core 20 is magnetized. Then, after the current stops and become zero, magnetic force generated by the current is removed. But some magnetic flux remains in the core 20 because of magnetic hysteresis effect. The remaining flux is defined as residual flux. The magnetic sensor 14 detects the residual flux, and consequently the sensor 30 measures an error current.
The measurement error can be corrected by storing fixed data corresponding to the error in ROM (Read-Only Memory), which is mounted on a current detection circuit of a current sensor, if a current flows in one direction. However, the error correcting method with ROM cannot be applied to a current sensor for measuring a battery current. This is because a battery current flows in both directions for charging and discharging a battery, and the residual flux density in the core 20 varies depending on the direction and magnitude of current. Consequently, it is difficult to correct a measurement error with ROM storing fixed data corresponding to the error.
The sensor 10 has another problem. When a large current of around several hundred amperes flows though the bar 12, magnetic flux density in the core 20 increases significantly and hysteresis effect is enhanced accordingly. Consequently, magnetic saturation occurs in the core 20 and the sensor 10 cannot measure an actual current.